This invention relates to a method for detecting/processing image information on an original film such as a roll of film negative which can detect and process image information with an image sensor having a relatively rough picture element density and which can detect an edge between imaged frames.
Photographic printing systems need to measure the density of an original film (e.g. a film negative sheet) in order to determine optimal printing exposure or correction amount. In the prior art, the average density of the film negative is photometrically measured with respect to its LATD (Large Area Transmittance Density) by means of photo sensors, such as photodiodes, provided near a light path in the photographic printing system. The prior art is defective, however, in that it can not accurately measure exposure or correction value because the image detection in the LATD method is intended to photometrically measure the film negative in average but not to precisely measure the image density across the whole surface of the film negative. The present applicant has filed applications which are laid-open under Japanese Laid-open Patent Nos. 154244/1985, 151631/1985 and 220325/1985 in order to solve the problem. But there arises a dilemma in that if the picture element density of an image sensor is increased, the costs for the image sensor as well as peripheral circuits will be pushed up and the time required for processing an exposure value, for example, is prolonged. For quick operation of the exposure value, for example, the picture element density of the image sensor is preferably to be relatively rough.
In a photographic printing system, it is further necessary to accurately position a frame of the original film on a photographic frame in order to suitably print images on a sheet of photographic paper. In the prior art, in the the case of the film sizes such as size 135, where frames and perforations do not correspond to a 1:1 relation, like 110 and 126 sizes do, for example, notches are cut on the sides of a sheet of the original film detected by photosensors and then used for positioning. This method, however, leads to trouble because the notches must be cut with utmost care in order to have an accurate correspondence with the frame positions. There is another positioning method which feeds a sheet of the original film according to prescribed distance, but positional deviations tend to accumulate to deteriorate the precision. Still another method proposes that photosensors such as photodiodes are arranged in correspondence to the shapes of frames to position the original film according to the states detected by photosensors and sequence of detection, but this method inconveniently requires a complicated structure and control algorithm. The method is further defective because the slit mask which is mounted in order to enhance resolution tends to lower the sensitivity.
In order to solve such problems, the present applicant has filed inventions related to methods under Japanese Laid-open Patent Nos. 196740/1985 and 185793/1985, but these methods are defective in that the density of these image sensors generally has to have a high resolution. This is because unless the edge between two imaged frames is detected at a high precision, the original film cannot be controlled accurately when being conveyed. The resolution may be increased by the method of pitch interpolation, but it takes a long time in image information detection because data is taken for every minute pitch and detected over a long span of frames. The method is detrimental to the speed of data processing as well as to the efficiency of works. It is especially inconvenient when the edges are detected for the film feeding because the whole feeding time amounts to a formidable extent.